Encoders (or resolvers) have been used in motor systems, such as brushless DC servomotors, to control the operation of the motor system. An encoder is used to provide position and speed information of a rotor of the motor system. This information is used by an external motor controller having electronics to control the operation of the motor system.
Rotary optical encoders such as housed encoders are commonly used in motor systems to provide the rotary position of the motor system. A rotary optical encoder typically includes a housing to support precision bearings and electronics, a shaft with a disk (e.g., an optical disk) having alternating clear and opaque patterns, a light source (e.g., a Light Emitting Diode), and an assembly of a photodetector and a mask. A beam of light produced by the light source is projected onto the optical disk, which is constructed of a clear material with opaque radial lines. When the optical disk rotates, the light beam passes through the clear areas but is blocked by the opaque areas so that the optical disk effectively modulates the light beam. The pulsed light beam is then received by the mask/photodetector assembly where electric signals are generated and provided to a motor controller.
Another type of rotary optical encoder, called a kit encoder, is also widely used in motor systems. Instead of having a separate housing, shaft, and bearings for the housed type rotary encoder, the kit encoder relies on the motor shaft, i.e., the disk is mounted on the shaft of the motor itself in the kit encoder. Other elements of the kit encoder such as the light source and the mask/photodetector assembly are mounted to the motor housing.
Kit encoders have significant advantages over housed encoders. In particular, overall system size and manufacturing cost is reduced because numerous components such as the encoder shaft and precision bearings are not required. However, the accuracy of the kit encoder suffers because the motor shaft on which the optical disk is mounted has limited precision. For example, the shaft on which the optical disk is mounted in the kit encoder is particularly subject to eccentric movement. The dimensional tolerances of the components can allow eccentric motion such as when the center of the disk pattern and the shaft center are not coincident. This occurs on all encoders, but is exaggerated on kit encoders because the motor shaft normally has poorer dimensional control than do the shafts of housed encoders. Varying shaft side loads (i.e., forces normal to the motor shaft) can translate the shaft center also causing eccentric motion. This eccentric movement creates errors in measuring the rotational position of the motor shaft. The errors are generated predominantly at a frequency of one cycle per shaft revolution of the motor and are approximately equal in magnitude to the magnitude of eccentric movement.
FIG. 1A is a schematic diagram of a conventional transmissive type encoder 10 showing simplified elements such as a light source 12, a disk 14, a mask 16 and a photodetector 18. FIG. 1B is a plan view of the encoder 10 of FIG. 1A without the light source 12 shown for simplicity, i.e., only the disk 14, the mask 16 and the photodetector 18 are shown as overlapped. As shown in FIG. 1B, a disk is patterned with radial lines pattern formed of alternately clear area and opaque radial lines. The mask is designed to have a line pattern as similar as possible to a relevant portion of the radial lines of the disk.
As the disk rotates with the light source turned ON, the light beam passes through the clear areas of the disk and are blocked by the opaque lines. Subsequently, the non-blocked light from the disk arrives at the mask. When the clear areas of the disk are directly over the clear areas of the mask, about 50% of the incident light in the patterned area gets through to the photodetector. However, when the opaque lines of the disk are directly over the clear areas, nearly no light will get through to the photodetector. The photodetector generates electric signals proportional to the strength of the received light beam, which varies approximately as a sine wave when the disk rotates at constant speed. These electric signals are processed by external circuitry to calculate the rotational position of the disk (i.e., rotor position). Most encoders sense an incremental position relative to an arbitrary starting point which is normally the position of the disk when power is applied. An optional reference mark may be placed on the disk to indicate position of the disk within a single revolution. The position of the encoder may then be calculated in relationship to that reference.
The position provided by a conventional encoder is corrupted by eccentric movement, i.e., small eccentric movement that causes the disk to translate as it rotates. As shown in FIG. 1C, there are three types of disk motion created by shaft rotation, i.e., disk rotation, normal translation and tangential translation. The disk rotation is the primary motion to be sensed, and ideally, the encoder would sense only rotation. However, the disk may be translated normal to the mask as well as tangential to the mask. Tangential translation causes the radial lines on the mask to move relative to each other just as rotation does. In fact, in most encoders, tangential translation cannot be differentiated from rotation. In such a case, tangential translation corrupts the encoder output. Note that normal translation does not cause the lines in the disk and mask to move relative to each other and so does not corrupt the encoder output. Eccentric motion is approximately circular, causing both tangential and normal translation. For example, if the disk were offset 0.002″ in the normal direction at zero degrees (e.g., at 3 o'clock position where the sensor 18 is located), the normal translation would be approximately 0.002 cos(θ) and the tangential translation would be approximately 0.002 sin(θ) where θ is the angle of disk. The tangential translation adds directly to the position output resulting in a feedback position approximately equal to the sum of the actual position and the tangential translation divided by the radius. Some applications may not tolerate these errors and thus a user might forced to use a higher precision encoder, for example, changing from a kit encoder to a housed encoder or increasing the precision of manufacture of a housed encoder. Such changes may increase the cost or size of the encoder, or require the user to accept other undesirable qualities of the position sensor.